The present invention generally relates to a copolymer resin for a photoresist used in the manufacture of electronic devices. The photoresist can be used with an ultraviolet ray such as KrF or ArF. More particularly, the present invention relates to processes for preparation of the photoresist, processes using the photoresist, and the photoresist itself. In an exemplary embodiment, the invention relates to a photoresist resin, wherein a mono-methyl cis-5-norbornene-endo-2, 3-dicarboxylate unit has been introduced to a norbornene-maleic anhydride copolymeric structure for a photoresist, usable in a lithography process using a KrF (248 nm) or ArF (193) light source which could be applied in 1 G or 4 G DRAM devices, and others; a process for preparation thereof; and a photoresist containing the same resin.
Various types of photoresists have been used or proposed. These photoresists often have a variety of characteristics or properties such as etching resistance, adhesiveness, and others. In general, etching resistance and adhesiveness with low light absorption at 193 nm wavelength are desirable for a copolymer resin for ArF. The copolymer resin also should be developable by using 2.38 wt % aqueous tetraethylammonium hydroxide (TMAH) solution. It is difficult to synthesize a copolymer resin, however, satisfying one or more of these properties. Many. researches have focused on studies on novolac-type resin as a resin to increase transparency at 193 nm wavelength and increase etching resistance. As an example, Bell Laboratories tried to introduce an alicyclic unit to the backbone chain in order to enhance etching resistance. The copolymer resin in which the backbone chain has norbornan, acrylate and maleic anhydride substituents, is, for example, represented by formula I: 
The copolymer resin of formula I, in which the maleic anhydride portion (A portion) used for polymerizing the alicyclic olefin group, however, is the only material to be polymerized with norbornene, the alicyclic unit, without absorbing ArF light having 193 nm wavelength. Accordingly, it generally cannot be used as a resin for ArF, because it is quite soluble in 2.38% aqueous TMAH solution. The solubility occurs even without exposure to create a xe2x80x98top lossxe2x80x99 phenomenon (the top of pattern being formed is in a round shape) which is generally seen in conventional photoresist patterning.
Thus, in order to prevent such phenomenon, the y-portion having a tert-butyl substituent should be increased. A relative decrease of the z-portion, which enhances sensitivity and adhesiveness with substrate, causes disadvantages in that the photoresist is removed from the wafer in conventional patterning so that the pattern cannot be formed effectively. Other resist products have been proposed, but these resist products can have numerous limitations such as difficulty in manufacturing, offensive odors, and the like.
From the above, it is seen that a photoresist having better adhesiveness and improved resolution is desired.
According to the present invention, the present inventors developed a maleic anhydride type copolymer resin comprising a 5-norbornene-2-carboxylic acid monomer represented by formula II as a main constituent, and filed a patent application (Korean Patent Application No. 97-26807 filed on Jun. 21, 1997), as an attempt to solve limitations of conventional photoresist products. 
Though a photoresist using the maleic anhydride type copolymer resin suggested by the above application is a polymer resin with high adhesiveness, sensitivity and excellent resolution, there is a problem in practical manufacturing, because one of the main constituents, 5-norbornene-2-carboxylic acid may generate a very offensive odor during the synthetic process. Thus, the present inventors have developed a novel photoresist having excellent resolution without causing the problem of offensive odor.
In a specific embodiment, the present invention provides a technique for limiting the offensive odor caused by conventional resist products. More particularly, the present invention provides a method of introducing a mono-methyl cis-5-norbornene-endo-2, 3-dicarboxylate unit instead of a 5-norbornene-2-carboxylic acid unit in the norbornene-maleic anhydride copolymer structure. The present method resolves, in part, any problem of emitting an offensive odor. In a preferred embodiment, the present method provides a resulting resist without substantially deteriorating resist sensitivity. The present photoresist also has characteristics such as excellent adhesiveness and resolution (0.13 xcexcm). The present resist can be obtained with easy control of constituent composition during the synthesis of photoresist resin to make the mass production possible.
Numerous advantages or benefits are achieved by way of the present invention over conventional techniques. In a specific embodiment, the present invention provides a copolymer resin comprising a mono-methyl cis-5-norbornene-endo-2, 3-dicarboxylate unit. In an alternative embodiment, the present invention provides a process for preparing the copolymer resin comprising a mono-methyl cis-5-norbornene-endo-2, 3-dicarboxylate unit. In still a further embodiment, the present invention provides a photoresist comprising the above norbornene-maleic anhydride copolymer resin, organic solvent, and photo acid generator. Still farther, the present invention provides a semiconductor element manufactured by using the photoresist comprising the above copolymer resin. These and other benefits are described throughout the specification and more particularly below.
The present invention relates to a copolymer resin for a photoresist comprising a mono-methyl cis-5-norbornene-endo-2, 3-dicarboxylate unit, which is represented by following formula III: 
where, R represents tert-butyl, tetrahydropyranyl, tetrahydrofuranyl or ethoxyethyl group, and the ratio x:y:z is (0.1-99%):(0.1-99%):(0.1-99%).
The copolymer resins according to an embodiment of the present invention preferably include norbornene-maleic anhydride copolymer resins represented by formulas IV to VII. 
In the above formulas, R, x, y and z are defined as above.
The copolymer resin of the present invention, represented by formula III, can be prepared by polymerizing the norbornene derivative of formula VIII, 2-hydroxyethyl-5-norbornene-2-carboxylate of formula IX, maleic anhydride of formula X and mono-methyl-cis-5-norbornene-endo-2, 3-dicarboxylate of formula XI in the presence of a radical initiator. 
In the above formulas, R represents tert-butyl, 2-tetrahydropyranyl, 2-tetrahydrofuranyl or ethoxyethyl or like groups.
In the preparation of the copolymer resin according to the present invention, norbornene derivatives of formula VIII are preferably selected from the group consisting of tert-butyl 5-norbornene-2-carboxylate, 2-tetrahydropyranyl 5-norbornene-2-carboxylate, 2-tetrahydrofuranyl 5-norbornene-2-carboxylate and 2-ethoxyethyl 5-norbornene-2-carboxylate.
The copolymer resins according to the present invention can be prepared by a conventional polymerization process such as bulk polymerization or solution polymerization. As a solvent, cyclohexanone, methyl ethyl ketone, benzene, toluene, dioxane and/or dimethylformamide may be used individually, or in a mixture. Polymerization initiators usable in the present invention include benzoyl peroxide, 2,2xe2x80x2-azobisisobutyronitrile (AIBN), acetyl peroxide, lauryl peroxide, tert-butyl peracetate, di-tert-butyl peroxide, or the like.
In the process for preparing the copolymer resin according to the present invention, general polymerization conditions including temperature and pressure of radical polymerization may be controlled dependent upon the property of the reactants, but it is preferable to carry out the polymerization reaction at a temperature between 60 and 200xc2x0 C.
The copolymer resin according to the present invention can be used in the formation of a positive micro-image by preparing a photoresist solution in which the resin is mixed with an organic solvent and a conventional photo acid generator according to a conventional process for preparing a photoresist composition. In the process for forming a photoresist pattern for a semiconductor element, the amount of the copolymer resin according to the present invention depends on the organic solvent or photo acid generator used, and the condition of lithography, but conventionally it is about 10 to 30% by weight on the basis of the organic solvent used in the preparation of the photoresist.
The process for forming a photoresist pattern for a semiconductor device by using the copolymer resin according to the present invention is described in detail here-in-below:
The copolymer resin according to the present invention is dissolved in cyclohexanone at a concentration of 10 to 30% by weight. A photo acid generator (0.1-10 wt %), such as triphenylsulfonium triflate, dibutylnaphthylsulfonium triflate, 2,6-dimethylphenylsulfonate, bis(arylsulfonyl)-diazomethane, oxime sulfonate or 2,1-diazonaphthoquinon-4-sultonate, is incorporated to the photoresist resin. The mixture is then filtered through an ultra-micro filter to prepare a photoresist solution. The photoresist solution is spin-coated on a silicon wafer to form a thin film, which is then soft-baked in an oven at 80-150xc2x0 C. or on a hot plate for 1-5 minutes, exposed to light by using far ultraviolet exposer or an eximer laser exposer, and baked at a temperature between 100xc2x0 C. and 200xc2x0 C. for 10 seconds to 60 minutes. The exposed wafer is impregnated in 2.38% aqueous TMAH solution for 1 to 30 seconds to obtain a positive photoresist pattern.